The invention relates to a process for the recovery of ammonia during the separation of nitrogen oxides from the flue gases produced in a firing plant during the combustion of materials, particularly fossil fuels. A reducing gas, preferably ammonia, is introduced into the flue gas flow in a reaction zone and as a result the nitrogen oxide gases are reduced to elementary nitrogen. The flue gas flow then undergoes washing, which is performed following the reaction phase. As a result of the wash water, the unreacted ammonia or ammonium salts, referred to as slip, are removed and subsequently the pH-value of the wash water enriched by ammonia and ammonium during the washing of the flue gases is raised to above 9.5 by introducing a liquor.
A process of the aforementioned type is known from the Applicant's EP-A-264 041. In this process, the ammonia dissolved in the wash water is desorbed in a further process stage and expelled from the wash water as gas. For this purpose an intimate contact is brought about between the wash water and the air by air introduced into a stripping column, so that the ammonia is transferred from the wash water into the air. This air-ammonia mixture can then be fed as combustion air into the firing plant, e.g. in the form of secondary air in refuse incineration plants and specifically into the flue gases.
This process is based on the idea that the reduction of nitrogen oxides in flue gases can be improved by the introduction of ammonia gas into the latter, as occurs in the known SNCR process (EP-A-79 171) or the known SCR process (U.S. Pat. No. 3,970,739), if it is possible to reduce the non-reacting ammonia proportion on increasing the degree of separation. This is brought about in the known process in that the flue gases are washed and the pH-value of the ammonium ion-enriched wash water is raised to a value higher than 10, e.g. by adding lime, so that the NH.sub.4.sup.+ ion absorbed and dissolved in the wash water of the wet washer or scrubber is converted by this neutralization into free, physically bound ammonia NH.sub.3. The free ammonia mixed with air passing out of the stripping column is sucked in by a blower, mixed with ambient air and blown as secondary air into the boiler.
Although this process makes it possible to achieve a high nitrogen oxide separation level with a simultaneously lower ammonia consumption, it means that the recovered ammonia can only partly be reused for the reduction of nitrogen oxides. This applies if the combustion plant is operated with air and if the secondary air is supplied at a point of secondary air injection where there are temperatures below 1000.degree. C. As a result of the high oxygen amount made available after the secondary air injection point, a considerable proportion of the ammonia burning cannot be avoided. If the temperatures are even higher, all the ammonia can burn, so that additional nitrogen oxides are produced with which the flue gases are additionally burdened.